Click Image to see larger version in my gallery 7 x 10 in - 18 x 25 cm. This one was done using black watercolour paint. If you look back a just couple of days to the 'Knife and Sharpener' you can see that it uses a mixture of Ultramarine Blue and Brunt Sienna (a strong reddy brown) to make the black.
Black paint, as a watercolour, is frowned upon by some 'purist'. To them back paint, as used here, is "bad", where as mixing your own black as in 'Knife and Sharpener' is "good".
What those 'purists', they aren't purists but that's another story, have done is to turn the very sound advice to beginners of - "Don't use back to darken other watercolours" into the nonsense of - "no watercolourist should ever use black".
What the painter saw.